Electric signaling system



E. M. S. M WHIRTER ETAL ELECTRIC SIGNALING SYSTEM April 3, 1951 Filed Jan. 2, 1947 F/ 65. 4 ur mpur FROM I g F G. 2 .5

OR 4 D I E 5 Sheets-Sheet 2 //VPU7' FROM F/G. 2,3 012 4 0 Attorney April 1951 E. M. s. MGWHIRTER ET AL 2,547,035

ELECTRIC SIGNALING SYSTEM 5 Sheets-Sheet 5 Filed Jan. 2, 1947 In ventors E/P/C M. 5'. MCWI/I T P FOL/1N0 H. DUN/V l t TE? W. LEN/VOA Attorney April 3, 1951 E. M. s. MOWHIRTER ETAL 2,547,035

ELECTRIC SIGNALING SYSTEM Filed Jail. 2, 1947 5 Sheets-Sheet 5 ATTORNEY Patented Apr. 3, 1951 UNITED STATES PATENT OFFICE ELECTRIC SIGNALING SYSTEM Application January 2, 1947, Serial No. 719,773 In Great Britain February 5, 1946 6 Claims. 1

This invention relates to electric signalling systems and has for its object an improved method of remote indication of the conditions of electric circuits.

The main feature of the invention comprises a circuit for translating an electric value into a teleprinte code.

The invention enables conditions at an unattended station to be teleprinted at regular and closely-spaced intervals to an attended station where a permanent record can be printed.

The invention will be particularly described in relation to certain of its many possible embodiments shewn in the accompanying drawings in which:

Fig. 1 shows a circuit for translating A. C. volts into D. C. volts preparatory to translation to teleprinter code,

Fig. 2 shows a circuit for translating A. C. amperes into D. C. volts,

Fig. 3 shows a circuit for translating the rate of operation of the change-over contact of a kilowatt hour meter into D. C. volts,

Fig. 4 shows a circuit for smoothing fluctuating D. C. voltages from the circuits of Figs. 1-3,

Fig. 5 shows a simple comparison circuit for comparing the D. C. volts from any one of Figs. 1-4 with a range of reference voltages,

Fig. 6 shows a pentode comparison circuit,

Fig. 7 shows a balancing amplifier for comparison purposes,

Fig. 8 shows a. modified balancing amplifier circuit,

Figs. 5 to 8 are alternatives to one another.

Fig. 9 shows an automatically operable potentiometer circuit for applying a range of D. C. reference potential to one of the comparison circuits, and means for controlling the transmission of a teleprinter code in accordance with the setting of the potentiometer when the comparison circuit operates, while Fig. 10 is a simplified form of Fig. 9 to explain the potentiometer arrangement more clearly.

Fig. 11 shows the means for controlling the transmission of a teleprinter code in accordance with the setting of the potentiometer when the comparison circuit operates.

Fig. 12 is a generalized schematic diagram explanatory of the principle of the invention.

For the purpose of carrying out the invention it will be described how the various electrical values, which are required to be translated into teleprinter codes, are first transformed into corresponding D. C. voltages. These voltages are fed to a. comparison circuit which on operationof a key compares them in turn with individual D. C. reference voltages from a plurality of such reference voltages which areavailable. The comparison circuit is arranged to operate first of all when a roughly equal reference voltage is applied to it. This operation results in the connection to the comparison circuit of a further plurality of D. C. reference voltages, the difference between adjacent voltages of this further plurality being much smaller than exists between adjacent voltages of that previously applied. A fine adjustment then automatically takes place, the comparison circuit operating again when two voltages applied to it are much more close to equality. The second operation of the comparison circuit results in the positioning of a switch on terminals indicative of the reference voltage which has been chosen as most closely corresponding to that resulting from the transformation into a D. C. voltage of the particular electrical value which it is desired to translate into a teleprinter code. It is the positioning of the switch, whereby circuits are completed for appropriate digit codings, which results in the transmission of the teleprinter code.

Various transforming circuits and several alternative comparison'circuits together with the circuit for provision of reference voltages and operation of the positioning switch, will now be described in some detail.

In general, the invention provides a method of converting either D. C. voltages or A. C. voltages of different amplitudes into coded impulse combinations, for example those of the 5-unit teleprinter code; so that the various signal levels can be transmitted to a distance and reproduced by conventional printing telegraph apparatus.

Fig. 12 shows in generalized schematic form, the more important parts of a system according to the invention. the input voltages to be coded are applied to a balancing amplifier, to gether with a reference voltage whose magnitude is automatically adjusted under control of two automatic switches such as are conventionally used in automatic telephone switching systems. Switch controls a coarse potentiometer and switch #2 controls a fine potentiometer. The potentiometer resistances are tapped to the fixed or bank contacts of the respective switches, and the wipers of these switches are arranged to be moved over their respective bank contacts until the reference voltage applied to the balancing amplifier equals the input voltage to be coded. The operation of the automatic potentiometer adjusting switches and the balancing of the amplifier are efiected mutually and automatically.

When the amplifier has become balanced, a relay BAR in the output of the amplifier releases, and. stops further operation of the automatic switches. Each of the switches has additional levels of bank contacts A, B, C, D, E, and respective wipers, the latter beingconnectable to a line so as to apply to the line'- successive code combinations of: oppo-. site polarity impulses which combinations are determined by the automatic setting of the said switches. It will be understood, of course, that the various bank contacts of the levels A, B, C, D and E of switch #I are connectedrtosuitable. positive and negative potentials sothat the par-u ticular potentials which are applied to. the five conductors Lt represent the coded combination of the tens digit of the desired input voltage level to be coded. Likewise the five wiresL willhave various combinations of positive and negative potential applied thereto determined by the position of the wipers of switch #2 on their respective bankcontacts A. B, C, D and Brand corresllqnd ing to the. units digit of the said input voltages to becoded. These combinations of coded im-. pulsesthen can be transmitted to any suitable recording machinewhich operates on the teleprinter or coded impulsecombination type. Preferably, however, they are connectedto the bank contactsof a third automatic, switch for the pur-' pose of sending sequentiallyuthe.various code impulses over a single Z-conductor line as will be described in connection with Fig. 11.

In Fig. l a transformer T is connected to the circuit carrying the potential to be vmeasured-at a convenient point and provides a suitable input for. a full-wave rectifier R which can be a metal rectifier or an arrangement of valve; rectifiers with smoothing elements consisting of resistance YA, vYB and condensersCA, CB deliveringa D.;C-.'

Fig, 3 is fundamentally different and is based on obtaining the reading from a kilovvatt-lriour meter such as is normally part of power station equipment fitted with a change-over contact which normally exists for the printometer circuit. The change-over contact lvicharges condenser CA from battery 0 in the position shown and discharges CA-into CB in its other position.

CA is a small condenser and CB is alarge condenser. Condenser CB'has aleak resistance X and the voltage across condenser CB arrives at a stable figure when the quantity which leaks away through XA between impulses equals the quantity put in by CA on each impulse. To obtain linearity it is necessary that CA shouldalways. supply the same quantity. It shouldtherefore always be charged to a voltage whichis 6 plus the voltage onCB. Condenser CB feeds the grid of valve V which is arranged as a cathode. fol-w, lower. Resistor YB provides a cathode voltage normally equal to the grid voltage. This cathode voltage and-supply e are connected in series.

Supply e is shown as a battery but would normally be a small rectifier circuit.

Linear output is obtained only if the cathode voltage exactly equals the grid voltage. This can be obtained for all practical purposes by making the cathode follower V a high gain amplifier. If theratio betweenthe cathode, voltage and the grid voltage is a similar result can be obtained by discharging condenser CA to k, the grid voltage, this can be efiected by connecting CA to a suitable tapping point on YB as shown in Fig. 3a.

The D. C. voltage, obtained across is applied to the comparison circuits.

Wherethesystemis being used to give readings of fiuctuating'vloads it is necessary to freeze the reading so that it remains constant during the time. takento translate into code. The circuit shown in Fig. 4 enables a condenser SC to be chargedfrom the output voltage from the circuit of Fig. 1, 2 or 3, via back contact CH, and then via front lcontactg Cfi; to be .appliedito the comparison circuit. Since the comparison circuitis an infinite inputresistance this arrangement-is.

sufficient.

The functionv of the. comparison circuit has already been briefly stated. The unknown input. voltage is compared in turn with a numlfser ofavailable reference voltages The application of a reference voltage equal to the input voltage, re-.

suits in the transmission of a teleprinter code corresponding to the value so determined: The comparison circuit thereiorerequires to be stable and to give a discrimination better-than onepart in a hundred. The circuitshown in Figure 5 in.

which-a sensitive differential relay is used cannot be .made to give the, accuracy required but can be'usedior the less pretentious type otcircuit.

The circuit shown in Fig. 6 uses anv amplifying.

valve V -to operate relayBAE In this casethe input voltage is applied first andpmakes the grid of the .valve negative well beyond anode. current cut-off. The -reference voltage, is then increased in steps untilthevalve commences to pass anode current v and finally, operatesrelay BAR. The valve requires a short grid base in comparison with the input and reterence. voltages and. can consist of. all C amplifier ofmore than one stage. Suitable resistances YA-.YE are provided. I p

The expression .gridbase used inthepreceding paragraph, refers to the range of grid :volt

ages between thatrequired to cut off anode current on the one hand, and that required to. produce the maximum; anode current which the valve can pass on the otherhandfor any iven anode voltage condition. A valvewith a short grid base is one having. only asmall difference.

for these two extreme grid voltages.

Fig. 7 shows a .balancingamplifier i and this is the arrangement-which: at present we .aprefer;

The input and the reference voltagesare applied to the grids of two valves VAaridVB which have a common cathode load. resistance The valves therefore operate-as cathode followers and thevalvewith the higher positive grid potential passes all the anode'current- If thegiid potentials are arranged graduallyto become equal, the anode current is shared between the valves; as

the grid potentials diverge. the-current swings to .PA', PE beingaused to feed-each stage- 0f "said A. twostage D; C.- amplifier comprising amplifier. Suitable load, bias and droppin resistances YA, YB, YC, YE, YF, YG are provided for the valves VB, VC and VD.

It should be noted that this circuit requires a power supply providing three connections, positive, earth and negative, and a pair stability is required in the ratio between the positive and negative sides. This is an easy requirement to meet with electronic stabilisation.

Fig. 8 shows an arrangement similar to Fig. 7 in which the cathode follower controls an oscillator valve VC so that oscillation starts as the reference voltage approaches the input voltage. When valve VB is conducting, the voltage drop across resistance YC reduces the anode potential available to valve VC, thereby preventing oscillation thereof. However, as soon as VB cuts off, the anode potential of VB rises and valve VC receives sufficient anode potential to commence oscillating. The A. C. generated by the oscillator is then amplified and operates relay BAR via an amplifying valve VD. Valves VC and VD are coupled by means of transformer LA. This circuit is included as it avoids the D. C. amplifier and the necessity for the stabilised supply.

Figs. 9 and show the balancing coder. A stabilised reference voltage which is 110% of the input voltage to the comparison circuit corresponding to a full-scale reading, is applied to resistances R forming a potentiometer connected to the contacts of one bank of a switch SA, and the voltage is tapped ofi this potentiometer and supplied as the reference voltage to the balancing amplifier, Fig. 7 or 8. Switch SA takes steps along its potentiometer bank contacts so as ultimately to control the setting up of the first tens digit of the code, and it stops on the contact next above the correct first digit. This stopping is effected by the relay BAR in the balancing amplifier (Fig. 8) a contact bar! of which is shown in the coder circuit, acting as a trip for the switch driving magnets SAM and SBM. Relay BAR when it attracts its armature to close with its right hand contact (Fig. 9), as well as stopping the switch by opening the energizing circuit of switch driving magnet SAM, operates a relay CO which connects a fine potentiometer consisting of a series of fine resistors r on switch SB over a contact e02 and samZ across two sections of the coarse potentiometer of switch SA, between the point at which the switch SA has been tripped and two steps further back. Potentiometer resistances R (comprising eleven sections) are connected between the bank contacts of level semi of switch SA in such a manner, that the first contact of sum! is connected between the second and third section of resistances R; the second contact of saml is connected between the third and fourth section of resistances R; the balance of the contacts being similarly connected to succeeding sections of the resistances R, until eleven contacts have been so connected. The same connections are duplicated in the second half of the 25 point switch saml so that even use is made of all bank contacts. An additional resistance RR is erially connected between the low potential end of potentiometer R and ground. The first contact of level cam! is connected to the junction point of resistance RR and the potentiometer R. The second contact of level samZ is connected between the first and second section of the potentiometer resistances R, and the third contact of level sam2 i connected to the first contact of level saml. The fourth contact of level sum! is connected to the second contact of level saml, and so on. It will thus be seen that the connections between level saml and sum! are effectively staggered by two, and the wiper of level samz is always two steps behind the wiper of level saml along the potentiometer resistances R. This reduces the value of the two steps from 2R to R and an additional resistance R is inserted at the bottom end of the potentiometer to make the total resistance correct. Since the wiper of switch sbml starts at the beginning of its potentiometer, this switching has effectively brought the potential down by one tens step, to balance the fact that switch SA trips on the step beyond the correct point. Switch SB will then hunt and trip on the next unit step.

If the reading is of the input full-scales reading, switch SA will run to a point corresponding to of the reference voltage whereupon it stops. The insertion of switch SB will reduce the 90% to 80% and switch SB will run to the 85% point.

The switches carry other levels to mark the teleprinter sender, and according to the strapping can mark any characters required. This operation will later be explained with reference to Figs. 9 and 11.

It is proposed to use more than 10 sections on switch SA so that normal full load readings will be figures approaching per cent, and over-load readings will give figures exceeding 100 per cent. The setting of switch SA can be used to mark both the hundreds digit when required and the tens digit. The appearance of a 3 digit number instead of a 2 digit number on the printed record draws attention to the incidence of over-load. The manner of producing the third digit of the code will be explained below.

The potentiometer shown in Fig. 9 comprises two single motion step b step wiper switches SA, SB of the kind used in automatic telephony, 1e switches being used for coarse and fine adjustment respectively, incorporating in well known manner steppingmagnets SAM, SBM, stepping interrupters AI, BI, contact banks and co-operating wipers saml sbml The only other equipment in the potentiometer consists of an electromagnetic contact-making relay CO also of the kind used in automatic telephony, and controlling five sets of contacts,three normally-open pairs of contacts col, 003, one normally closed pair (:04, and change-over contacts 005, a test key KT, a hoining key l-lKl, 2, and change-over contacts bari of a test reiay BAR such as that associated with the balancing amplifier test device (Fig. 8)

Potentiometer resistances R are connected between the bank contacts of level 5am! of switch SA. It will be seen that the complete coarse tentiometer consists of HR, two below the normal positions of sum! and nine between consecutive contacts of semi. The bank contacts of samZ are cross-connected to bank contacts of saml, the cross-connections being staggered by two contact so that in efiect, so far as the R potentiometer is concerned, wiper samZ is two steps behind saml. The potentiometer connections are duplicated in the second half of the 25 point switch.

The fine potentiometer comprises ten resistances r, such that 5r=R. Nine of the resistances r are connected between bank contacts of sbml, with the tenth in series between the other nine and. wiper saml. The lower end of the fine poa" tenti'ofneter is connected independently oil-wiper." som via el -normally open contactl'co2vtoi'wiper; sa'm22 Earth: is connected via;- a: l'esistanceu RR shunted"by; normally-.-closed icontactsscozi to. the lower end ofr thetcoarseilpotentiometer while battery' is connected to: the. upper? end thereof: Wiper sbml isconnected to theba'lanceuarnplie fier: of Figs. -7:or '8 to provide the :reference :volte age; The eleven coarse resistances R allow; the: potentiometer to 7 record: any? percentages 1 up to: 110%. It will-beclear of course that 'thisalrange'i can be extendedhy'employingzz z greater numbers of coa'rse resistors Wipers and' banksasa'mil, 5,: 851112 indicate the percentage setting-of the potentiometer: as? a. whole; sam l giving a hundreds' digit 1 lwhenthe reading is 109%.01 .over," sam3 give :the tens .di'git, and'sb mfi the units digit. The:elevenlresistancesJ R allow the potentiometer. to recordiin percentages up to:1101%; thus couldaof course: be'eX 'tendedi- The appearance of the hundreds digitcanlbei used as' an indication oi.aoverl'oad ofthe'electric circuit under.examination and whose. condition is; being remotel indicated If the: potentiometer? is to "control the. transmission off the. percentages of the full-scale value by teleprinterzcodazcone tactsof otherllevelssofxthe said switches"SA v and SB will .be connected; .asiishownin Fig; lot-for in stance; Dnly one possible method :of code esend .ing is contained'in' the .followingdescription and. indicatedzin-thei drawing; but use may. also be... made, for'example; of the teleprinterrcodecsend ing equipment described lil'liUIlltEd States -Patent: No; 2,433,362.

With :test KeyKT thrown and the balance'am plifier contact bar-l in the positionshown the" stepping circuit' for magnet SAM is closed, and wipers saml, 2 etc. are stepped upwards. Reference' potential f connected to the balance" am- 40 plifier comparison circuit between wiper SGW'H and earth: The sum 5 connection to the amplifier is via the fine potentiometer butthis has no appreciable eflect as the grid oiiampl-ifiervalve VD takes no 7 current;

When saml haspassedresi'stances sufllcientto raise the reference voltage above the voltage unden comparison, and wip'erssam'l; 21ers; for instance, onthe 7th contact position shown in" Fig. 9 'valve. VD is caused-to'conduct; (Fig 6, 7 50 or "8). andtherebythe amplifier rela'y BAR-Zoper'-- ates change-over contactsibar i which open the stepping circuitfiof SAM andv energise relay-00 which locks via e03. Contacts 0022 close con necting wiper samZ 'to the bottom oflthe fine po tentiometer so that, as will be. clearly-seen? from:

Fig. 9, the fine potentiometer of .10'r- 2R:is. con nected in parallel with the 2R; portion off the coarse potentiometer between saml and saw-222,

giving a resultant resistance of B: At thei-samee 60 troducing the additional R at the hottomof the 70' coarse potentiometer: in the: manner described above;

Duesto: the eff ective 1'eduction in the potential via sbm I z to the; balance:amplifier:the:relay BARv- SW-itChf-SB. so that-the setting of sWitohes'SA; SB

nowinclicate the percentage rcomparison between i the reference voltage and the voltage under con parison.

It hastalrea'dy been 'indic-atedrhow theupotentie ometer! reacts to an 851%. requiredvoitagecand..it. will: be." clear: that although there is: originally of the available.110% referencevcita'ge (re-- ferre-drtoi. full-scale input to; the. comparison :circuit) below th'e firstzterminal this position'or the: wipermust result. in the printingof the ten's digit 1.

It will :be seen thatif the voltage: under com!- parison is less than 20% ofthe maXimumrefer-- ence potential, SA will not step, but CO Willbef immediately operated and thepotenjal connected tOiSYJinliWill bere-ducedsto10%= value; Pro-- videdl'the voltage under comparisonlis-between 1 3% and-20%, S will startsteppingi. l

The? circuit is not designed" to evaluate 'volt'- ages of'less than 10% value, althcugh rhis could of course be done by arranging. that whenttheitest circuitt-(i. e; Figs. 1, 2.. and 3) is in its zero indi-' cating condition thecompari'soncircuit balanced. This may in some circumstances he done by raising or loweringiithe referencellevel of the output from the. test circuit;

The first terminal contacted by the wiper sb'm'l is responsible for the I printing of the tens-printer code'representing-theunits digit '0. The tenth and'eleveinh will both print 9. This is-nec-- essary. because if the inputvo'ltages to the com-- parison. circuit are. between, say; 39% and :40"% of I the full-scale reading (or 49% and 50% etc;) the following: operation must take place. SA will first' step to the thir'd' terminal, Les-the one at which" 40% oi the input voltage-i tapped and Which marks. a tendigit of 3'. The; various operations already detailed tn'en take pl-aceand SB -steps; At the tenth terminal the reference voltage will still not besufiicient" to operate the comparison circuit relay I BAR for although l this is the terminal normally-responsible for printing 9 theinput voltage now being applied to the comparison circuit is greater than 39 but less than 402 SB' steps to the -eleventh"terminal therefore; the" voltage is 110W" sufiicient to trip BARFbut -it is arranged'forthisterminalalso to print'the units digit 9 The 39 40 indicationis-now able tobe correctly coded.

It will be seen'tha-t common testmeans' serve bot-h the coarse" and fine potentiometer-s in turn without modification, modification taking place Within the potentiometers to allow'for automatic adjustment of both potentiometers in turn.

The method of translating the positioning of' the potentiometer wipers; which takes place-automatically-'once'the ke KT has been operated, Will 'noW be explained with" reference to Figs; 9 and 11. The operation orthepotentiometer level of the switches SA and SB will also-of course have positioned samii 8 and smbz 6.? The terminals onall'these levels in thealatter has released;.andscoritacts'bari*re 75.? except semi/8 have either positive or negative" polarities on them so providing the five potential code for both tens and unit digits. Any setting of SA and SB therefore is transferred to a binary coding over these levels and placed on five wire sets which pass to the terminals of another automatic telephone type switch SC on its level scml. Referring back to the eighth level samB of the switch SA we see that to certain terminals are arranged to complete a circuit for a relay H. These terminals correspond to the positions at which the potentiometer value to be transmitted contains three figures, i. e. is 100% or over of the full-scale indication.

Looking now at Fig. 11 it will be seen thatthe relay H has five change-over contacts hl I15. Normally the circuit X, giving the characteristic code potentials for a space are connected through the change-over contacts to scml terminals but the operation of rela H connects the circuit Y thereto so arranging for the sending of the hundreds unit 1.

A pendulum relay, or other timing means, is provided for timing the code sending and this has two contacts 137! and pr2 one normally closed whilst the other is open.

When the comparison circuit has completed its operations bdTz and 005 are closed and slim! is in an off-normal position. Providing m2 is closed therefore SE will operate and look over se2. On the next closure of brl following the closure of se! the magnet SCM will be energized and SC will step on each subsequent opening of prl. The wiper scml then scans its terminals finding thereon the coded representation of the value which is to be transmitted to the line together with the inter-digit polarities which it is standard practice to send. After completing its travel SC completes a circuit over scmZ for the unlocking of the relay SE so providing for the release of SC and its return to its home position.

With SC standing again in its home position ke KT may be restored, releasing CO whilst over HKI and samQ and HKZ and sbmfl SA and SB may be stepped to their next home positions. A circuit may be arranged for this restoration to be carried out automatically or it may be done manually, say in response to an indicating lamp showing the return of SC to its home position after transmitting the teleprinter code.

What is claimed is:

l. A system for converting input electric signals of different energy content into telegraph code signal combinations comprising a source of telegraph code potential; an outgoing line, a source of reference potential in graduated steps; means for comparing said input signals with said reference potential including a balancing network, said network, comprising grid-controlled electron discharge means, said discharge means having a short grid base in contrast to the range of energy content of the input signals to be compared, means for applying said input signals to said discharge means in a direction to prevent the flow of space current in said discharge means, first switch means of the plural contact, plural bank type, having different of the contacts of selected of said banks connected to different of the steps of said reference potential, and different of said banks having selected contacts thereof connected to said code potential in a predetermined polarity sequence, said first switch means under control of said discharge means and adapted to sequentially apply different steps of said reference potential to said discharge means in a direction opposite to said input signals, additional switch means for sequentially scanning said different contact banks of said first named switch means, said additional switch means coupled between said first named switch means and said line whereby said code combinations are applied to said line, to indicate the varying energy content of said input signals.

2. A systemas claimed in claim 1, wherein said balancing network further comprises a plural section potentiometer, said potentiometer connected in shunt across said source of reference potential and wherein, the different contacts of said selected contact banks of said first named switch means are connected to different sections of said potentiometer in a predetermined sequence.

3. An electrical signalling apparatus, as claimed in claim 1, wherein said electron discharge means comprises two grid-controlled electron discharge valves having a common cathode load resistor, means for applying said input signals to the grid of a first one of said valves in a positive sense, means for applying the reference voltage to the grid of said second valve over said first named switch means, relay means connected to the output of said second valve, said relay adapted to be responsive when the reference voltage equals the energy content of said applied input signals, said first named switch means under control of said relay means, whereby operation of said switch means i halted when said reference voltage equals the energy content of the input signals applied to the grid of said first valve.

4. A system as claimed in claim 3 wherein said comparing means includes a second plural section potentiometer and said first switch means comprises a pair of switches of the plural contact, plural bank type, having plural cooperating wipers therefor the first of said pair of switches havin different of the contacts of selected banks thereof connected to different of the steps of said reference potential, the second of said pair of switches having diiierent of the contacts of a selected bank thereof connected to different of the sections of said second potentiometer, the wipers of the selected banks of the first of said pair of switches connected to opposite ends of said second potentiometer, the wiper associated with the selected bank of the second of said pair of switches connected to the grid of said second valve, and said relay means connected to the output of said second valve comprises a two position electromagnetic relay, said relay adapted selectively to control the operation of a different of said pair of switches in each position of said relay.

5. A system as claimed in claim 1, wherein said means for applying said input signals to said discharge means includes means for rectifying the alternating current component of said input signals.

6. A system as claimed in claim 1 wherein said electron discharge means further comprises a further electron discharge valve, a resonant circuit coupled to said last-named valve, means to couple said last named valve to said comparing means, said valve adapted to remain in normally non-oscillating condition, and means for causing said last named valve to oscillate when the potential from said reference source approximates the energy content of the input signals applied to said comparing means and means coupled to I 12 :saiddast namedxvalve tefio ntrol speration of llumber Name gDate moth-said-switch means. 2;l10,0l5 Fitz Geraldmnunu Mar.;1,1938

ERIC MALCOLMSWIFT MCWHIRTEJR. 2,297,744 Larson ..July, 16,f19-40 RQLAND'HARRIS:DUNN. 2,217, 17? Gulliksen 0.01).:8, Z1940 WILLIAM LENNOX. :5 2,237,713 "Russell :Apr. 53,1941 .2,24=1,743 :Schoene Mayrl3, 1941 ,R-EFERE'FEES QZTE "2,432,826 smith Dec. 16, 1947 'Thefollowing references :are of record in the F R GN PATENTS me of thls patent 110 Number Country Date UNITED STATES PATENTS 266,798 .GreatBritain Feb. 28, 1927 Number Name Date 15602382 .Chubb 7.- 'MQct. -.12, 1926 l,778;827 :Evans '-Oct. 21,1930 

